This invention relates to the removal of hydrogen from gases by using a catalyst. More particularly, the invention relates to a device and a process for removing hydrogen from gases containing hydrogen with a catalyst arrangement composed of a substrate which is coated with a material that catalyzes the oxidation of hydrogen while releasing heat. The invention can be used for removing hydrogen from gases and mixtures of gases containing, in addition to hydrogen, oxygen, steam and aerosols.
For example, the hydrogen can escape into the atmosphere of the safety vessel, which contains oxygen, or into the atmosphere of a pressure suppression system of a nuclear reactor. To eliminate the escaping hydrogen, the prior art device uses metals which have a high degree of hydrogen absorbency, even at low hydrogen partial pressures in gas mixtures.
To prevent oxidation, the metals are coated with a hydrogen-permeable protective layer. This coating makes it possible to use a material for the protective layer which acts as a catalyst to oxidize the hydrogen with whatever oxygen is present, to form water.
The hydrogen-absorbent metal can be either self-supporting or be a layer on a substrate. For use as a safety device that is required only in the event of an accident, provision is made in the prior art device for the substrate bearing the catalyzing protective layer to be mounted inside a closed container that opens only in the event of an accident. Typically the container opens in response to pressure. In one embodiment of the prior art device, the container has a rupture disk at its upper and lower ends which breaks in the event of an accident. Inside the container is a package of the coated metal(s) that absorb hydrogen. The surrounding gas mixture flows into the container after the rupture disks are broken, creating a convective gas flow in the container from its bottom to its top.
In another embodiment of the prior art device, a film made of the coated hydrogen-absorbent metal is stored rolled up in a container. The container is hung from a ceiling and opens in the event of an accident so that the rolled-up film can unroll and produce a large surface area for contact with the gas mixture.
A similar device is known from German Patent Application No. DE-A 37 25 290. That publication teaches a material that is especially well suited as a catalyst and with which a lattice-type or sheet-type substrate is coated on one or both sides. The coated substrate is stored folded or rolled up in a gas-tight housing that opens only in the event of an accident so that the substrate can then unfold or unroll. The substrate and the catalyst materials disclosed in that document are well suited for use within the scope of the present invention.
During a core meltdown in a nuclear reactor pressure vessel (RPV), a temperature rise of up to 2400.degree. C. can occur, with large quantities of fission products and of structural materials being released into the atmosphere of the containment vessel. The result is a mixture of steam and gases in which aerosol particles can be suspended, typically with a weight concentration of up to twenty g/m.sup.3. The term "aerosol" is used herein in a broad sense to mean a suspension of liquid or solid particles in a gas. Thus, for example, in the low-pressure path at the beginning of the interaction between the melt and the concrete, one to three tons of dispersed material can be suspended in the air in the containment vessel. By far the largest percentage by weight, commonly more than 95%, is nonradioactive. However, most of the radioactive substances are bonded to aerosol particles. The release of hydrogen during reactor accidents, mentioned at the outset, coincides in time with this release of aerosols.
Prototype tests have shown that, commonly, the release of steam occurs practically simultaneously with the beginning of a core meltdown accident, while the release of hydrogen and, simultaneously therewith, the release of aerosols, take place only after a certain delay. When large quantities of steam are present and the flow is strong, the catalytic reaction to remove hydrogen proceeds more slowly. The reaction rate increases exponentially with temperature. It is only after a sufficiently high temperature is reached at the surface of the catalyst that sufficient convective flow develops to prevent the aerosol particles contained in the gas mixture from being deposited on the surface of the catalyst. This prevention is reinforced by the constant generation of steam at the surface of the catalyst, which occurs at correspondingly high temperature and conversion rates. However, as long as the temperature of the catalyst in the initial phase is not yet sufficiently high, aerosol particles and grease particles contained in the steam can settle on the catalyst surface, reducing the effective catalyst surface and thereby considerably reducing the action of the catalyst.
Thus, it is an object of this invention to provide a device of the type recited above and in which the deposition on the surface of a catalyst of aerosol particles and of grease particles contained in the steam is largely prevented, and the temperature required to achieve a high reaction rate is reached quickly.
The invention as described and claimed below achieves this and further objects that will become apparent from the following description.